Generally, permanent magnet rotating electrical machines are classified into two types. One is a surface-permanent magnet rotating electrical machine having permanent magnets adhered to an outer circumferential face of a rotor core and the other is an interior permanent magnet rotating electrical machine having permanent magnets embedded in a rotor core. For a variable-speed drive motor, the interior permanent magnet rotating electrical machine is appropriate.
With reference to FIG. 20, a configuration of a conventional interior permanent magnet rotating electrical machine will be explained. Along an outer circumference of a rotor core 2 of a rotor 1, rectangular hollows are arranged at regular intervals, the number of the rectangular hollows being equal to the number of poles. In FIG. 20, the rotor 1 has four poles, and therefore, four hollows are arranged to receive permanent magnets 4, respectively. Each permanent magnet 4 is magnetized in a radial direction of the rotor 1, i.e., in a direction orthogonal to a side (long side in FIG. 20) of the rectangular section of the permanent magnet 4 that faces an air gap. The permanent magnet 4 is usually an NdFeB permanent magnet having a high coercive force so that it is not demagnetized with a load current. The rotor core 2 is formed by laminating electromagnetic sheets through which the hollows are punched. The rotor 1 is incorporated in a stator 20. The stator 20 has an armature coil 21 that is received in a slot formed inside a stator iron core 22. An inner circumferential face of the stator 20 and an outer circumferential face of the rotor 1 face each other with the air gap 23 interposing between them.
Known examples of such a permanent magnet rotating electrical machine are described in “Design and Control of Interior Permanent Magnet Synchronous Motor,” Takeda Yoji, et al., a document of Ohm-sha Publishing (Non-patent Document 1) and Japanese Unexamined Patent Application Publication No. H07-336919 (Patent Document 1). An example of a high-output rotating electrical machine having an excellent variable-speed characteristic is a permanent magnet reluctance motor. Examples thereof are described in Japanese Unexamined Patent Application Publication No. H11-27913 (Patent Document 2) and Japanese Unexamined Patent Application Publication No. H11-136912 (Patent Document 3). Further, there is an interior permanent magnet motor employing alnico magnets whose magnetic force is changed. Examples thereof are described in U.S. Pat. No. 6,800,977 (Patent Document 4) and Weschta, “Schachung des Erregerfelds bei einer dauermagneterregten Synchronmaschine”, ETZ Archiv Vol. 7, No. 3, pp. 79-84 (1985) (Non-patent Document 2).
The rotating electrical machine of the Non-patent Document 2 is a permanent magnet motor employing alnico magnets whose flux amount is changed. This related art may demagnetize the alnico magnets but it hardly magnetizes the alnico magnets to an original magnetized state. The rotating electrical machine mentioned in the Patent Document 4 is a flux-concentration-type interior permanent magnet motor employing alnico permanent magnets. This rotating electrical machine is a modification of the rotating electrical machine described in the Non-patent Document 2, and like the rotating electrical machine of the Non-patent Document 2, applies a magnetic field to change the flux amount of the alnico magnets. The rotating electrical machine of the Patent Document 4 is a simple alnico magnet motor, and therefore, its output is insufficient. When torque is produced, the alnico magnets are demagnetized by a load current, to decrease the torque. To generate sufficient torque with the alnico magnets whose energy product is small, the alnico magnets must be thick in a magnetizing direction. To magnetize such thick alnico magnets, a very large current must be passed. Then, it will be difficult to magnetize the permanent magnets, or change the flux amount of the permanent magnets.
In the permanent magnet rotating electrical machine, a permanent magnet always generates constant linkage flux to increase a voltage induced by the permanent magnet in proportion to a rotation speed. Accordingly, when a variable-speed operation is carried out from low speed to high speed, the voltage (counter electromotive voltage) induced by the permanent magnet becomes very high at high rotation speed. The voltage induced by the permanent magnet is applied to electronic parts of an inverter, and if the applied voltage exceeds a withstand voltage of the electronic parts, the parts will cause insulation breakage. It is necessary, therefore, to design the machine so that the flux amount of the permanent magnet is below the withstand voltage. Such a design, however, lowers the output and efficiency of the permanent magnet rotating electrical machine in a low-speed zone.
If the variable-speed operation is carried out in such a way as to provide nearly a constant output from low speed to high speed, the voltage of the rotating electrical machine will reach an upper limit of a power source voltage in a high rotation speed zone, not to pass a current necessary for output because the linkage flux of the permanent magnet is constant. This greatly drops output in the high rotation speed zone and the variable-speed operation will not be carried out in a wide range up to high rotation speed.
Recent techniques of expanding a variable-speed range employ flux-weakening control such as one described in the Non-patent Document 1. The total linkage flux amount of an armature coil is the sum of flux by a d-axis current and flux by a permanent magnet. The flux-weakening control generates flux with a negative d-axis current to reduce the total linkage flux amount. The flux-weakening control makes a permanent magnet of high coercive force operate in a reversible range on a magnetic characteristic curve (B-H characteristic curve). For this, the permanent magnet must be an NdFeB magnet having a high coercive force so that it may not irreversibly demagnetized with a demagnetizing field produced by the flux-weakening control.
In the flux-weakening control, flux produced by a negative d-axis current reduces linkage flux and a reduced portion of the linkage flux produces a voltage margin for an upper voltage limit. This makes it possible to increase a current for a torque component, thereby increasing an output in a high-speed zone. In addition, the voltage margin makes it possible to increase a rotation speed, thereby expanding a variable-speed operating range.
Always passing a negative d-axis current that contributes nothing to an output, however, increases a copper loss to deteriorate efficiency. In addition, a demagnetizing field produced by the negative d-axis current generates harmonic flux that causes a voltage increase. Such a voltage increase limits a voltage reduction achieved by the flux-weakening control. These factors make it difficult for the flux-weakening control to conduct a variable-speed operation for the interior permanent magnet rotating electrical machine at speeds over three times a base speed. In addition, the harmonic flux increases an iron loss to drastically reduce efficiency in middle- and high-speed zones. Further, the harmonic flux generates an electromagnetic force that produces vibration.
When the interior permanent magnet motor is employed as a drive motor of a hybrid car, the motor rotates together with an engine when only the engine is used to drive the hybrid car. In this case, a voltage induced by permanent magnets of the motor increases at middle or high rotation speed. To suppress an increase in the induced voltage below a power source voltage, the flux-weakening control continuously passes a negative d-axis current. The motor in this state produces only a loss to deteriorate an overall operating efficiency.
When the interior permanent magnet motor is employed as a drive motor of an electric train, the electric train sometimes carries out a coasting operation. Then, like the above-mentioned example, the flux-weakening control is carried out to continuously pass a negative d-axis current to suppress a voltage induced by permanent magnets below a power source voltage. The motor in this state only produces a loss to deteriorate an overall operating efficiency.
A technique to solve these problems is disclosed in Japanese Unexamined Patent Application Publication No. 2006-280195 (Patent Document 5). The technique described in the Patent Document 5 relates to a permanent magnet rotating electrical machine capable of conducting a variable-speed operation in a wide range from low speed to high speed and improving efficiency and reliability. This machine includes a stator provided with a stator coil and a rotor having a low-coercive-force permanent magnet whose coercive force is of such a level that a magnetic field created by a current of the stator coil may irreversibly change the flux density of the magnet and a high-coercive-force permanent magnet whose coercive force is equal to or larger than twice that of the low-coercive-force permanent magnet. In a high rotation speed zone in which the voltage of the machine exceeds a maximum power source voltage, a current is passed to create a magnetic field that magnetizes the low-coercive-force permanent magnet in such a way as to reduce total linkage flux produced by the low-coercive-force and high-coercive-force permanent magnets, thereby adjusting a total linkage flux amount.    Patent Document 1: Japanese Unexamined Patent Application Publication No. H07-336919    Patent Document 2: Japanese Unexamined Patent Application Publication No. H11-27913    Patent Document 3: Japanese Unexamined Patent Application Publication No. H11-136912    Patent Document 4: U.S. Pat. No. 6,800,977    Patent Document 5: Japanese Unexamined Patent Application Publication No. 2006-280195    Non-patent Document 1: “Design and Control of Interior Permanent Magnet Synchronous Motor,” Takeda Yoji, et al., Ohm-sha Publishing    Non-patent Document 2: Weschta, “Schachung des Erregerfelds bei einer dauermagneterregten Synchronmaschine”, ETZ Archiv Vol. 7, No. 3, pp. 79-84 (1985)